FIG. 1 is an elevational view of a direct-to-sheet, continuous-web, phase-change ink printer. Systems of this type were proposed, for example, in U.S. patent application Ser. No. 11/773,549 filed Jul. 5, 2007, and Ser. No. 11/696,954 filed Apr. 5, 2007, herein incorporated by reference in their entirety.
A very long (i.e., substantially continuous) web W of “substrate” (paper, plastic, or other printable material), supplied on a spool 10, is unwound as needed, propelled by a variety of motors, not shown. A set of rolls 12 controls the tension of the unwinding web as the web moves through a path.
Along the path there is provided a preheater 18, which brings the web to an initial predetermined temperature. The preheater 18 can rely on contact, radiant, conductive, or convective heat to bring the web W to a target preheat temperature.
The web W moves through a printing station 20 including a series of printheads 21A (C), 21B (M), 21C (Y), and 21D (K), each printhead effectively extending across the width of the web and being able to place ink of one primary color directly (i.e., without use of an intermediate or offset member) onto the moving web. As is generally familiar, each of the four primary-color images placed on overlapping areas on the web W combine to form a full-color image, based on the image data sent to each printhead through image path 22. It should be understood that the images may be printed on the web W by any ink jet printing process or any printing operation as known to one of ordinary skill in the art.
The ink directed to web W may be a “phase-change ink” (i.e., the ink is substantially solid at room temperature and substantially liquid when initially jetted onto the web W), although, other types of inks may also be used.
Associated with each primary color printhead is a backing member 24A, 24B, 24C, 24D, typically in the form of a bar or roll, which is arranged substantially opposite the printhead on the other side of web W. Each backing member is used to position the web W so that the gap between the printhead and the web W stays at a known, constant distance.
Following the printing zone 20 along the web path is a series of tension rolls 26, followed by an image sensor 28 to detect defects in the one or more images formed on the web W. In embodiments, the image sensor 28 may determine whether the image is accurately printed.
The web W may be moved to a position adjacent to the coating station 29. The coating station 29 may apply or may transfer a coating onto the side(s) of the web W after the printing operation. In some implementations, application of the coating by the coating station 29 to the side(s) of the web W may be identified as a pre-finishing step within a continuous solid ink jet print process. “Pre-finishing step” refers to a step in a print process which may be completed prior to exposing ink on the web W to an ink spreading device or ink spreading procedure.
Following the coating station 29, there are one or more “midheaters” 30. The midheater 30 can use contact, radiant, conductive, and/or convective heat to bring the web W to the target temperature. The midheater 30 brings the ink placed on the web to a temperature suitable for desired properties when the ink on the web is sent through the spreader 40.
Following the midheaters 30, along the path of web W, is a “spreader” 40, that applies a predetermined pressure, and in some implementations, heat, to the web W. The function of the spreader 40 is to take what are essentially isolated droplets of ink on web W and smear them out in a controlled manner to make a continuous layer by pressure, and, in one embodiment, heat, so that spaces between adjacent drops are filled and image solids become uniform. In addition to spreading the ink, the spreader 40 may also improve image permanence by increasing ink layer cohesion and/or increasing the ink-web adhesion. The spreader 40 includes rolls, such as image-side roll 42 and pressure roll 44, that apply heat and pressure to the web W. Either roll can include heat elements such as 46 to bring the web W to a temperature in a range from about 35° C. to about 80° C.
The spreader 40 can also include a cleaning/oiling station 48 associated with image-side roll 42, suitable for cleaning and/or applying a layer of some lubricant or other material to the roll surface.
Following the spreader 40, the printer in this embodiment includes a “glosser” 50, whose function is to change the gloss of the image (such a glosser can be considered an “option” in a practical implementation). The glosser 50 applies a predetermined combination of temperature and pressure, to obtain a desired amount of gloss on the ink that has just been spread by spreader 40. Additionally, the glosser roll surface may have a texture that the user desires to impress on the ink surface. The glosser 50 includes two rolls (image-side roll 52 and pressure roll 54) forming a nip through which the web W passes.
Following passage through the spreader 40 and glosser 50, the printed web can be imaged on the other side, and then cut into pages, such as for binding (not shown).
The sensor 28, discussed above, may advantageously be a full width array (FWA) sensor 28 provided along the media path to sense and control the registration between the various print heads, color uniformity and/or color consistency of the inks. While a single sensor 28 is shown to be located prior to the midheater 30, it will be readily appreciated that the sensor 28 may be provided at other locations along the media path and/or multiple sensors may be provided.
The resulting two-dimensional image that is imaged by sensor 28 may be used determine the location of the centers of microscopic area of ink drops and how much ink is contained in them (also referred to as intensity or amplitude). These measurements are analyzed, for example, by software, to determine characteristics indicating mislocations of the print heads or any drops that may be missing or are incorrectly sized.
The optical contrast characteristics of the colors with respect to the substrate are critical for accurate sensing of the above features. Currently, a white light source, for example, fluorescent or LED, is utilized as an illuminator with the sensor 28 to provide a wide range of contrast for the various colored inks, with the largest contrast for black ink and the smallest contrast for yellow ink. As with many monochrome imaging systems, though, certain low contrast colored inks, and in particular yellow, make it difficult to obtain accurate measurements.